Solar heating systems have been around since nearly the turn of the century. Early systems, such as those shown in U.S. Pat. Nos. 1,250,260 to Wilcox and 1,425,174 to Cartter et al., however, suffered a general lack of sturctural integrity and lack of efficiency. With the price of fossil fuels very low, solar systems of this era were commercially unsuccessful.
At present, the solar water heaters for both residential and commercial uses are becoming more and more popular. This is primarily due to two trends in recent years. First, the costs of electricity and natural gas have increased dramatically and are continuing to escalate. Second, the integrity and energy efficiency of solar water heaters is rising due to improved engineering of the systems and components. Together, these trends make solar water heaters cost competitive in an expanding geographical area.
Solar water heating systems may be broken down into two general types; active and passive systems. Active systems are generally more expensive and include pumps and elaborate electronic controls to circulate the water and working fluid through the various components. Conversely, passive systems rely on the laws of nature, such as the laws of thermodynamics and gravity to provide circulation. The greatest appeal of passive systems, such as the present invention, lies in their simplicity and attendant ease of installation and low maintenance costs. These assets translate directly into low initial cost, reliability, low maintenance, and high performance.
Passive systems, however, are not without their disadvantages. They are, in fact, at a distinct disadvantage in northern climates where freeze protection is critical. Thus, if the fluid is not heated sufficiently by the solar collector, it may become stagnant and freeze resulting in loss of the heating function and possible damage to the system.
In extremely hot weather and/or in times of little or no hot water use, stagnation is again a problem with temperatures of the working fluid approaching those of a solar oven. At such high extremes of temperature, the solar collector glazing may yellow, and the working fluid may turn acidic and corrosive, and in some extreme instances, steam is formed causing a vapor lock and/or an explosion.
Therefore, in order to extend the free use of passive solar heating systems globally, there is a need to improve their function by simple, efficient means.